source: http://www.securityfocus.com/bid/3078/info

An input validation error exists in version 3.0.0 of the SSH daemon (sshd) running on Unix platforms.

It may be possible for remote users to log in to accounts for which there are two or less characters in the password field of the system password file. Due to the nature of the problem, it may be possible to log in to a vulnerable system using such an account with any password. This may lead to further system compromise. 

#!/usr/bin/perl
#
# A local SSH 3.0.0 vulnerability scanner for the 
# SSH Short Password Login Vulnerability, BugtraqID: 3078
#
# Note: You must have superuser access on the system to scan it.
#
# usage: ./ssh3.pl <host>
#        Optional: -e turn off error
#                  -h specify a different /etc/shadow file
# (Options must come before host name)
# 
# Written by hypoclear hypoclear@jungle.net - http://hypoclear.cjb.net
#
# This and all of my programs fall under my disclaimer, which
# can be found at: http://hypoclear.cjb.net/hypodisclaim.txt


use IO::Socket; use Getopt::Std;
getopts('h:e');

die "\nusage: $0 <host>\n\tOptional: -e turn off error\n\t\t  -h specify a different /etc/shadow file\n\n" unless @ARGV > 0;
if (!defined $opt_h)
 { $opt_h = "/etc/shadow";
 }

$out = &bannerGrab($ARGV[0],22);
sysread $out, $message,100;
close $out;

if (($message =~ /3.0.0/) || (defined $opt_e))
 { print "Running SSH 3.0.0, checking for vulnerabilities...\n\n";
   open(SHADOW, "<$opt_h") || die "Cannot open $opt_h!\nNote: You must have superuser access to run this script.\n\n";
   while(<SHADOW>)
     { $name = $_;
       $name =~ s/:.*$//;
       $_ =~ s/^.*?\://;
       $_ =~ s/:.*$//;
       $name =~ s/\s//g; $_=~s/\s//g;
       push(@name,$name);
       push(@hash,$_);
       push(@lnnum,$cnt++); $cnt++;
     }
   close(SHADOW);

   foreach $hash (@hash)
     { @chars = split(//,$hash);
       foreach $char (@chars)
         { $count++;
         }
       if ($count <= 2)
        { print "$name[$line]\t(line $lnnum[$line]) may be vulnerable!\n";
          $vulnFlag = 1;
        }
       $count=0; $line++;
     }
   if ($vulnFlag != 1)
    { print "No accounts appear to be vulnerable.\n";
    }
 }
else
 { if (!defined $opt_e)
    { print "You are not running SSH 3.0.0.\n";
      die "If you feel that this is an error run with the -e option.\n";
    }
 }
print "\n";


sub bannerGrab
{ $host  = gethostbyname($_[0]) || warn "cannot connect to $ARGV[0]\n";
  $port  = getservbyport($_[1], 'tcp'); 
  $haddr = sockaddr_in($_[1], $host); 
  socket(OUT, PF_INET, SOCK_STREAM, getprotobyname('tcp')) || warn "$!\n"; 
  connect(OUT, $haddr) ;
  return OUT; 
}

